1. Field of the Invention
This invention relates to a fluid delivery system, and more particularly to a hygienic fluid delivery system, the construction of which requires careful planning and analysis to ensure that the resulting system has minimal stagnancy of flow during operation and that bacteria growth is minimized if not eliminated entirely.
2. Description of the Prior Art
The types of fluid delivery systems hereinafter described are typically provided in a wide variety of plants, including Research & Development facilities generally, laboratories, silicon wafer manufacturing plants, breweries, pharmaceutical manufacturing facilities, and any installation where a plurality of sources of sterile water or other liquid may be required in a number of different locations remote from a supply tank for storing the water or other liquid.
Although the following description is provided with almost exclusive, reference to hygienic fluid delivery systems which deliver so-called Water for Injection (WFI), Purified Water (PW) and the like, it will be instantly appreciated that the invention has far wider application and may be applied to deliver any liquid to a predetermined location remote from a storage vessel through distribution pipe work.
It is also to be appreciated by the reader that the word “sterile”and cognate expressions as used hereinafter is not to be construed in its literal sense and includes liquids having a bacteria, germ or other contaminant content reduced beneath a desired level so that the said liquids are safe or suitable for a particular procedure.
Current systems for the delivery of sterile fluids, in their simplest form, consist essentially of a storage vessel supplied intermittently or continuously with a sufficient volume of pre-sterilized fluid. A system of steel pipe work is routed through the premises, for example a clinical laboratory, wherein WFI is required, said pipe work being for the most part conventionally concealed in the ceiling of each room through which said pipe work passes and descending from the ceiling only in those locations either where operatives are likely to regularly require a source of WFI or alternatively in locations conveniently accessible by a number of operatives working proximate said location. The pipe work is routed through said premises and ultimately returns to the storage vessel to return any excess fluid thereto.
There are a number of important factors which must be taken into account when designing a sterile fluid delivery system, but the most important is that the system as a whole must generally preclude any localized stagnation of fluid, either in the pipe work or the storage vessel and be free from crevices or similar areas where bacteria could become trapped and thus allowed to proliferateAccordingly the pipes are firstly commonly welded together using a very costly technique known as Tungsten Inert Gas (TIG) autogenous welding which ensures that the butt joints between adjacent sections of pipe are secured to one another without introducing unwanted, contaminants into the passageway within the pipes and ensuring that the join is as smooth as possible internally. Furthermore the interior surface of the various pipes which are joined together throughout the system is important in that said interior surface must be as smooth as possible and any bends in the pipes must preclude the formation of eddy currents during fluid flow there through. It will be appreciated that eddy currents give rise to localized volumes of fluid which are effectively stationary, and thus the temperature of these volumes can quickly drop to a level at which bacteria most readily thrive with the result that the sterility of the system as a whole is prejudiced.
Secondly, the operating temperature of the system is adjusted and maintained to ensure that any bacteria (for example mesophilic bacteria such as gram-negative pseudomonas commonly present in water) are either prevented from multiplying or are actually eliminated. A common temperature for WFI systems is 80° C. and to prevent any gradual reduction of the fluid temperature over time, a heater is commonly connected into the system.
It is to be mentioned that PW and other hygienic fluid systems can be operated at ambient temperatures, so much greater care and attention needs to be given to the construction of these types of systems which on account of the operating temperature are much more prone to bacterial proliferation.
Thirdly, it is important that a turbulent, as opposed to a laminar flow characteristic is developed within all sections of the pipe work again to minimize the risk of bacteria proliferation. For example, in both laminar and turbulent fluid flows, it is well known that the velocity of the fluid immediately adjacent a solid surface is minimal if not zero, whereas the velocity of fluid remote from such a surface is much greater. Hence the majority” of the volumetric “flow through a pipe is achieved through the middle of the pipe whereas only a comparatively small percentage of flow is attributable to the fluid moving proximate the interior surfaces of said pipe. This slow moving or stagnant fluid has the tendency to cool and thus not only are the conditions for bacteria proliferation improved adjacent to the interior surface of the pipe, but the fact that the fluid is either moving very slowly or not at all further increase the likelihood that bacteria will find a site on the pipe surface to germinate.
Turbulent fluid flows within sterile fluid delivery systems are desirable because velocity profile of the fluid proximate the interior surface of the pipe increase significantly more sharply than that of an equivalent laminar flow and the risk of bacteria proliferation and germination is thus mitigated. Biofilm formation on the internal surfaces is discouraged by such fluid velocities.
However, it is well known in fluid dynamics that the existence of turbulent flows within pipes depends on, among other things, the diameter of the pipe, and the velocity of fluid flow there through. In general, to the development of turbulent flow in pipes of a larger diameter requires a significantly larger fluid velocity than required to establish turbulent flow in smaller diameter pipes.
Fourthly, it is necessary to ensure that the storage vessel containing the WFI is recharged over a predetermined period of time, for example every two hours. Moreover, the system operates continuously so that the storage vessel is being continually emptied and simultaneously recharged to avoid any stagnation of fluid therein, and the time period is merely an indication of the length of time which would be taken to empty to the storage vessel completely under normal operating conditions without any simultaneous recharge.
It will also be appreciated that the systems with which this invention is concerned may have many tens of outlets or so-called off takes usually connected in series as an entire laboratory or building may need to be served by a single fluid delivery system. The diameter of the pipes commonly used in such systems may be of the order of 1-2Y2 inches (25-64 mm), and to ensure a turbulent flow within such a pipe the flow velocities are typically between 1-3 m/s.
In order to develop such a flow velocity, substantial and thus costly pumping apparatus is required, and when it is also considered that a number of different off takes may be in use simultaneously, dynamic control of this pump and/or alternatively some means of pressure regulation is required. Conventional Fluid delivery systems must possess an ability to deliver fluid through a number of offtakes opened simultaneously while nevertheless operating satisfactorily when none of the off takes are in use, for example overnight. The inherent disadvantage of series-type systems, in which a plurality of off takes are connected in series such as described, is that the opening of more than a few off takes simultaneously can have detrimental effect both on the flow characteristics and the ability to draw water at the correct flow and pressure at the various user off takes. The number of off takes which can be opened simultaneously in a system expressed as a percentage of the total number of off takes in a system is known as the diversity.